JCAT is the software platform for the Market Design Competition (CAT) under the umbrella of the Trading Agent Competition (TAC). The CAT Competition supports multiple marketplaces that run in parallel and compete against each other to attract traders and make profit. JCAT includes the CAT game server, trading strategies and marketplace selection strategies to configure trading agents, i.e., traders, and a parameterized, reference implementation of market mechanism that enables entrants to get started easily.